The image shows extensive effacement of the normal tubulointerstitial architecture and replacement by a dense, plasma cell rich infiltrate. While this finding is consistent with a chronic, active tubulointerstitial nephritis the image also shows a unique pattern of fibrosis. Within the inflammation, bands of collagen are present that create the effect of a nesting pattern or what some have argued is a storiform or ‘birds-eye maple’ pattern. This unique pattern of fibrosis is characteristic for IgG4-related disease as is true in this case (see IgG4 immunohistochemical stain below). Of note, IgG4 is a patchy disease which can often be mass forming. This is important to remember as small biopsy samples often show near total fibrosis. However, this finding may not be representative of the kidneys as a whole and these patients often respond well to treatment.